The overall objective of this research effort is the elucidation of the functioning of the enzyme cytochrome c oxidase in its normal, intracellular environment in brain tissue. The enzyme catalyzes the reduction of O2 to H2O as the final step in respiratory chain function and concommitant oxidative phosphorylation. By far the greatest part of our information on cytochrome c oxidase was derived from in vitro studies on purified or semi-purified preparations. However, recent studies have indicated that the enzyme behaves differently in vivo. The question that is raised concerns the consequences of the observed differences between the in vivo and in vitro characteristics in terms of enzyme function, oxidative metabolism, organ function and cell survival. Since many of the differences have been observed in brain tissue and since cerebral function and survival are most exquisitely dependent on oxidative metabolism, the intact mammalian brain is the chief organ to be studied. With differential spectrophotometry the redox state of the enzyme in the intracellular milieu will be determined under a number of test conditions ranging from hypo- and hypercarbia to hypo- and hyperoxygenation and from inhibition to stimulation of its activity.